Radical design work is being undertaken on the sequels to two of Britain's biggest recent rail projects, Crossrail and the Channel Tunnel Rail Link.

Boris Johnson's Transport for London team has been discreetly working on possible routes for Crossrail 2, under head of planning Michelle Dix, and London's mayor made a typically robust argument in favour of the project only last week. While the original £15bn line will link the west of the capital to its eastern half by 2017, the follow-up route would run from north to south.

Transport Secretary Justine Greening has asked the team at High Speed Two to ensure a better link-up between HS2 and its predecessor, the HS1 line that runs from London's St Pancras Station to the Channel Tunnel entrance in Kent. The Government sold HS1 to a Canadian consortium back in 2010 in order to raise money to plug holes in the UK's debt-ridden public finances.

The two pieces of work have a common link because the Government wants both HS1 and HS2 and Crossrail 2 to link-up properly. Ms Dix believes that HS2, which would link Euston Station to Birmingham and eventually the North-west, would only make sense for the capital if it was also linked to a second Crossrail line.

Similarly, officials want HS2 passengers to be able to link directly on to the existing HS1 line without changing trains. However, HS1 starts at St Pancras and it is just not feasible for the second high speed link to go there.

One outlandish idea that is understood to have already been dismissed was for a travelator between Euston and the nearby St Pancras, but the HS2 team is now concentrating on a track link to come later along the existing line. This might involve changing the type of track that has been proposed for HS2.

On the Crossrail work, a source said: "What they're doing is looking to change the Crossrail 2 route to accommodate HS2 by starting it at Euston. But then you've got to do something about Euston, even with the current plans to redevelop the station, as it won't be able to cope with Crossrail 2 as well as HS2."

A HS2 spokesman said: "It has always been the Government's intention that the design of the route will be further developed as part of the Environmental Impact Assessment process."